Back-projection screen

ABSTRACT

According to the invention, elements  71  distributed over one and the same diopter allow both macrocollimation and microfocussing by reflecting the light rays projected onto this screen; refractive elements  72  may also be distributed over this diopter.  
     The screen according to the invention has but a single main optical component  7 ; it is therefore very inexpensive to manufacture.  
     The invention applies especially to very compact projection devices and to cases in which the projection angles of incidence are high.

[0001] The invention relates to image backprojection screens of the typethat are used in television backprojectors.

[0002]FIG. 1 shows a system for displaying images on a backprojectionscreen 1, said system comprising, apart from the screen itself, at leastone image source (not shown) and optical means for generating the imageof this source on the screen through at least one objective 2; documentU.S. Pat. No. 3,791,712 discloses a system of this type.

[0003] The image source may, for example, be formed by the front face ofa cathode-ray tube or by a matrix of electrooptic valves that areformed, for example, from liquid-crystal cells, this matrix then beingcombined with a light source.

[0004] In the case of a laser projector, the system for displayingimages on a projection screen comprises a source of laser rays, meansfor modulating the emission intensity of this source, and means forcomplete scanning of the modulated ray coming from this source over thesurface of the screen; the scanning means may be based on rotatingmirrors as described in document U.S. Pat. No. 6,020,937.

[0005] Each pixel of the image formed on the screen thereforecorresponds, for example, to one element of the matrix of liquid-crystalcells or to the impact of the laser ray at the moment of scanning and ofmodulation.

[0006] To obtain image display systems that are more compact, it isgeneral practice to place mirrors in the optical path between the screenand the objective 2 or the laser source; FIG. 1 of document U.S. Pat.No. 4,512,631 describes such a compact system; according to othervariants, these mirrors may be replaced with holographic componentshaving the same function.

[0007] Finally, to display color images, these systems may comprise asmany image sources as primary colors, these being placed so as tosuperpose images of different primary colors on the same screen 1.

[0008]FIG. 2 shows the image backprojection screen 1, which comprises,the rear, means for macro-collimating the rays coming from the objective2 or from a laser source and, at the front, means for enhancing thedisplay contrast in ambient light.

[0009] The term “macro-collimation means” means optical means suitablefor directing the rays coming from the objective 2 or from a lasersource approximately in the same direction perpendicular to the plane ofthe screen; thus, when the optical axis of the envelope of the raysincident on the screen is perpendicular to this screen (zero angle ofincidence), these means are generally formed by a Fresnel lens 3.

[0010] The means for enhancing contrasts are generally formed by amatrix of black bands 4, as shown in FIG. 2.

[0011] It is general practice to place, between the Fresnel lens 3 andthis matrix of black bands 4, means for microfocussing the parallel rayscoming from this lens 3, these being suitable both for making the majorportion of the light flux pass between the black bands 4 and fordirecting the light toward the aid capable of observing the images to bedisplaced; this provision makes it possible, in combination with theblack bands, to enhance the contrast while minimizing the emissivesurface area of the screen, and provides a light-scattering function;these microfocussing means are generally formed by a lenticular sheet 5,as shown in FIG. 2, comprising in this case both microlenses 51 on therear face and microlenses 52 on the front face, these being placedbetween the bands 4 of the black matrix.

[0012]FIG. 3 shows schematically a partial cross section through thescreen of FIG. 2; the incident rays coming from an objective or from alaser source (these not being shown) are deflected by refraction througheach prismatic element 31 of the Fresnel lens 3 into a beam of parallelrays all orthogonal to the screen; each microlens 51 of the lenticularsheet 5 then makes the parallel rays that it receives between the bands4 of the black matrix converge.

[0013] If the angle of incidence on the screen 1 of the rays coming fromthe objective 2 or from the laser source is very high, that is to sayvery far from the normal to the screen as shown in FIG. 4, themacro-collimation means may be formed by prismatic elements operating byreflection, as described in document U.S. Pat. No. 4,003,080 (seeespecially FIGS. 3 and 4 of that document); FIG. 5 appended here is apartial cross section through the screen 1 provided with suchmacro-collimation means formed from prismatic elements 32 operating bytotal reflection or by reflection off a metal surface.

[0014] Whatever the configuration, the backprojection screens thereforecomprise, on the one hand, macro-collimation means 2 generally formedfrom prismatic elements 31; 32 and, on the other hand, microfocussingmeans 5 generally formed from microlenses 51, 52.

[0015] Such screens are therefore expensive to produce, not only becausethey comprise two separate optical components, to be manufacturedseparately, for example a Fresnel lens on the one hand and a lenticularsheet on the other, but also because these two separate elements must beaccurately assembled and aligned; such multiple manufacturing andassembly steps are economically penalizing.

[0016] The object of the invention is to remedy this drawback.

[0017] For this purpose, the subject of the invention is abackprojection screen to be placed in the field of an envelope ofprojection light rays, characterized in that it comprises elementsdistributed over one and the same diopter that makes it possible both tomacro-collimate and microfocus said rays by reflection.

[0018] Since the same elements allow both macro-collimation andmicrofocussing, instead of using, as in the prior art, on the one hand aFresnel lens for macro-collimation and on the other hand a lenticularsheet for microfocussing, only a single optical component is used tocarry out the two functions at the same time, which represents asignificant economic advantage.

[0019] Since a single diopter is used, the same surface fulfils thefunctions previously provided by two separate optical components; sinceall the optical elements of the screen relating to the macro-collimationand microfocussing functions are distributed over the same surfaceaccording to the invention, the screen according to the invention may beproduced by conventional methods, such as those used to manufactureFresnel lenses or lenticular sheets, such as for example injectionmolding or compression molding; the screens according to the inventionare therefore particularly inexpensive to manufacture.

[0020] Like the elements distributed over one and the same diopteroperating by reflection, the screen according to the invention isparticularly well suited to very oblique angles of projection incidenceand makes it possible to obtain a very compact projection device, evenwithout using mirrors; thus, the angle of incidence of the optical axisof the envelope of the incident rays with the plane of the screen may beeasily greater than or equal to 70° C.

[0021] Preferably, said reflection is a total reflection, as opposed tospecular reflection of a metal surface.

[0022] According to a variant of the invention, aside from the opticalelements already mentioned relating to the macro-collimation andmicrofocussing functions, said diopter includes elements that contributeto the collimation and/or focussing function by refraction.

[0023] Preferably, this diopter corresponds to the rear face of a mainoptical component of said screen.

[0024] The screen according to the invention can be used in anybackprojection image display system, especially in systems comprising atleast one laser source.

[0025] The invention will be more clearly understood on reading thedescription that follows, given by way of non-limiting example, and withreference to the appended drawings in which:

[0026]FIGS. 1 and 4 are simplified diagrams of backprojection displaydevices;

[0027]FIG. 2 is a perspective view of a screen according to the priorart of the device of FIG. 1;

[0028]FIG. 3 is a partial schematic cross section of the screen of FIG.2;

[0029]FIG. 5 is a partial schematic cross section of a screen accordingto the prior art of the device of FIG. 4;

[0030]FIG. 6 is a partial schematic layer of a screen according to oneembodiment of the invention; and

[0031]FIG. 7 is a rear perspective view of the main optical component 7of the screen of FIG. 6.

[0032] To simplify the description and bring out the differences andadvantages of the present invention over the prior art technique,identical references will be used for elements that provide the samefunctions.

[0033] The incident rays in the field of which the screen according tothe invention is placed come, for example, from a projection objectiveor from a laser source (these not being shown).

[0034] Referring to FIG. 6, which shows a partial cross section of oneembodiment of the screen according to the invention, having here on itsfront face a matrix of black contrast bands 4, a succession of diopterelements 71 distributed over one and the same surface or diopterorients, by reflection, the rays of the incident beam (or envelope ofthe rays) in a direction that is generally centered about the normal tothe screen and focuses portions of this beam so as to make the rays passbetween the bands 4 of the black matrix of the screen and to orient themtoward the aid capable of observing the images displayed on the screen;thus, placed opposite each gap between the black bands 4 is a diopterelement 71 which fulfils, at least partly, the two functions ofcollimation and focussing simultaneously; as shown in FIG. 6, tomacro-collimate and microfocus the entire incident beam, a succession ofdiopter elements 71 is placed side by side, these being joined by othersurface elements 72, in such a way that the set of elements 71, 72 formsthe rear surface of the screen according to the invention; the surfaceelements 72 here are flat and oriented orthogonally to the direction ofthe incident rays, so as not to deflect the direction of propagation ofthese rays; thus, the backprojection screen is formed only from a singlemain optical component 7, the rear face of which corresponds to a singlediopter that combines the elements 71, 72 and the front face of whichbears here the matrix of black bands 4; according to a variant of theinvention, this main optical component 7 may include lenticular elementsbetween the black bands, similar to the elements 52 in FIG. 2.

[0035] The optical component 7 of the screen according to the inventionmay be produced by conventional methods, such as those used tomanufacture Fresnel lenses or lenticular sheets, such as for exampleinjection molding or compression molding; the screens according to theinvention are therefore particularly inexpensive to manufacture, sincethe functions previously provided by two separate components, to bemanufactured separately and then assembled, are now provided by a singlecomponent.

[0036] As shown in FIG. 6, since the diopter elements 71 act byreflection, the angle of incidence of the light rays in the field ofwhich the screen is placed is very high, that is to say very far fromthe normal to the screen.

[0037] According to a variant of the invention that is not describedhere in detail, the surface elements 72 also participate in thecollimation and/or focussing function by refraction; for example, theseelements, again planar, are no longer placed orthogonally to thedirection of the incident rays; for example, these elements are nolonger planar and have a shape suitable for participating in thecollimation and/or focussing function; according to this variant, thediopter formed by the rear face of the main optical component 7 of thescreen 1 then acts by refraction at the surface elements 72 and byreflection at the surface elements 71.

[0038] Advantageously, the diopter formed by the rear face of the mainoptical component 7 of the screen 1 completely fulfils the collimationand focussing functions simultaneously.

[0039] It may therefore be seen that the screen according to theinvention is particularly well suited to the cases in which the angle ofincidence of the optical axis of the envelope of the rays projected ontothe plane of the screen is greater than or equal to 70°.

[0040]FIG. 7 shows schematically the arrangement in the form of circulararcs of the diopter elements 71, this arrangement being suitable for theangle of incidence of the rays striking the same diopter element to beconstant over the entire width of the screen; such an arrangement isalso described in FIG. 7 of the document U.S. Pat. No. 4,674,836 in thecase of conventional screens; preferably, the black bands on the otherface of the optical component 7 have the same circularly arcuate shapeso as to be placed between each diopter element over the entire width ofthe screen.

[0041] The screen according to the invention may advantageously be usedin any backprojection image display system, especially very compactsystems; it may in particular be used in laser projectors.

1. A backprojection screen (1) to be placed in the field of an envelopeof projection light rays, characterized in that it comprises elements(71) distributed over one and the same diopter that makes it possibleboth to macro-collimate and microfocus said rays by reflection.
 2. Thescreen as claimed in claim 1, characterized in that said elements aredistributed as circular arcs.
 3. The screen as claimed in claim 1 or 2,characterized in that said reflection is a total reflection.
 4. Thescreen as claimed in any one of claims 1 to 3, characterized in thatsaid diopter furthermore includes elements (72) that collimate and/orfocus by refraction.
 5. The screen as claimed in any one of thepreceding claims, characterized in that said diopter corresponds to therear face of said screen.
 6. A backprojection image display systemcomprising a screen as claimed in any one of the preceding claims, atleast one image source and optical means for generating the image ofthis source on the screen through at least one objective that generatessaid envelope of projection light rays.
 7. The image display system asclaimed in claim 6, characterized in that the angle of incidence of theoptical axis of said envelope in the plane of the screen is greater thanor equal to 70°.
 8. The image display system as claimed in claim 6 or 7,characterized in that the angle of incidence of the rays striking thesame diopter element (71) is constant over the entire width of saidscreen.